Recommendation Weight on Top of Speaker

I don't know much about the Concertos. If your bookshelf speaker has relatively thin cabinet wall, you can cut two pieces of 1/2 inch to 3/4 inch plywood, about the same size as the side wall, place one piece on each side and clamp them down with a pair of C clamps. That will definitely clean up the mid bass. The draw back is that it doesn't look very good.

maybe these would work ?
http://www.mapleshaderecords.com/audioproducts/brassweights_hub.php

Whatever you do, don't let your signif other put a plant on top, no matter how "good" it looks or sounds!!!

the speakers are fine with nothing on top. the sonus faber cabinets are superb and solid

Weight can be beneficial when utilized properly. Exact amounts must be determined by experimentation. I happened to own some old VPI bricks which were and are one of the best solutions for speaker dampening. Mapleshade heavy hats are also excellent and look cool as well!